The present invention relates to a reserve power system comprising a reserve power machine for the generation of power, consumers, several elevator drives consisting of an elevator hoisting motor and a frequency converter controlling it, and a distribution network connecting the reserve power machine, consumers and elevator drives.
When elevator service in the event of a power failure is to be ensured, business premises, hotels, hospitals and high-rise residential blocks are usually provided with an elevator system in which a separate control centre, often implemented with relays for simplicity, assigns operating turns to different elevators at least within an elevator bank and often to all elevators in the building. The elevators are allowed to start out by turns, generally towards the ground floor. The next elevator is given permission to depart either after the previous one has reported arrival at destination or after the lapse of a fixed length of time from the departure of the previous elevator. In the latter case, however, care is taken to ensure that the reserve power available is not exceeded by accident. In most cases, the relays of the motor drives are cross-connected so that only one elevator is able to run at a time.
In addition to a so-called main group intended for personal transport, typical business premises also contain other elevators, such as freight elevators and parking elevators. The main group elevators carry most of the elevator traffic and load. A typical mid-range building also has several elevator banks for personal transport, e.g. one at each end of the building, and several freight and parking elevators. In this context, `mid-range elevator` refers to elevator applications in buildings having about 10-20 floors. If a middle sized building has several elevator banks for personal traffic, a normal basis of design for a situation where elevator operation relies on reserve power is that one elevator in each bank for passenger traffic can remain in normal operation on reserve power after the rescue phase. In a building of this size, the stairs are still usable as an alternative to elevators in case of a power failure.
From the above-mentioned principle of designing the reserve power for only one elevator in each elevator bank, it follows that rescuing is very slow. A rule of thumb often applied is that at most 12 elevators are connected to the same reserve power system, from which it follows that in case of a power failure passengers have to wait for their turn to get out for a length of time ranging from a few minutes to a few tens of minutes.
In larger buildings with more than 20 floors, it is not sensible to design the reserve power on the basis of having only one elevator in each elevator bank operable with reserve power. Therefore, the reserve power capacity for such buildings has to be so designed that, relatively speaking, a larger number of elevators can be operated by reserve power. Although the reserve power centre must feed other equipment as well, elevators are generally the largest load in respect of both power and above all starting current, which in the case of elevators is a basis of design.
To achieve a good level of service, a large reserve power application is needed, resulting in fairly high costs for the building. It should be noted that a relatively large portion of the costs is associated with noise, smoke and oil leakage protection of the reserve power machines rather than electric generators. Therefore, the total costs are significant.
In market areas where power failures are very common, the reserve power system must be nearly perfect. This is because in a situation where reserve power is used, passengers behave in an unpredictable way and it often occurs that passengers do not evacuate the car according to instructions, with the result that the next elevators have to wait longer than they are assumed to. Instead of using a centralized reserve power system, elevators have been equipped with individual battery driven reserve power units that, to reduce the costs, are often used only to drive the elevator to the next floor, where the passengers are allowed to leave the car. This solution, too, is expensive because e.g. the door control system requires a separate supply consistent with the nominal mains voltage. Moreover, this arrangement involves high battery maintenance costs, among other things.